You're into a very tricky area that's very complex, even in economics. What is normally done in a cost-benefit analysis extending well into the future is that you discount future costs and benefits to the present by what we would normally call an interest rate, but what they would call a time preference discount rate. So you're getting into something that actually is a fairly arbitrary thing. I've seen studies in government that have had a discount rate applied of over 10% and I've seen them apply at 0%.
That's one of the questions that one has to know when looking at a program with long-term implications. Crime bills are like that; building a nuclear power station is like that, with enormous long-term implications, as we're realizing these days. And education itself is very important, but then again, you have great difficulty in defining the benefits 20 years from now for education.